Northwestern acquired Russell in 1999 for an undisclosed sum, though analysts estimated the deal was worth $1.2 billion at the time. According to Northwestern, Russell oversees almost $260 billion in assets, largely for institutional investors.

"Russell has been a good investment for us," said John Schlifske, chairman and CEO of Northwestern Mutual, in a statement. "Russell's operating results have made significant contributions to our financial results over the years. When you look at this sale price and the income produced for us since we bought Russell in 1999, you get a rate of return well in excess of equity indices over that period."

Meanwhile, Russell has scaled back its exchange traded products offerings. It decided in late November that the Russell Equity ETF, its last existing ETF offering, did not generate the projected level of interest it wanted. Russell will halt any investment into the fund in January and close it by Feb. 6, 2015.

While Russell is liquidating the fund, it does not mean Russell is abandoning the ETF market, the firm stated in a release. Russell continues to offer ETFs in the Australian market and believes the active U.S. ETF market still presents viable and important investment opportunities.